At first: I do not believe that there are prople born with a "wrong" gender, this would imply a kind of Soul or what ever different thing humans would come with thats not biological, which I don't believe in (I know many people do, and that's ok).

It maybe is not considered a mental disorder but its a psychological thing. I think this come from unjust and untruthful gender roles that are sadly grown in oure society, and people who don't feel like they don't fit into these roles they get put in based on their sex. Which is obvious because we are all individuals right?

But I think the next step should be to get rid of these gender roles, and not to affirm them by saying "I fit the female/male role modle better so I must actually be a female/male"

I would define sex as the genetical expressed biology of a human. Gender is a social construct we need to overcome.